1. Field of the Invention
The present invention relates to a trapping technique of allocating in an image, which is configured by allocating a plurality of figures having a relative upper and lower position order to their respective predetermined positions, a predetermined figure to a boundary portion at which two different colors contained respectively in the plurality of figures are adjacent each other.
2. Description of the Background Art
There has heretofore been known the technique of forming an image on a printing paper by multicolor printing with the use of a plurality of press plates, which correspond to a plurality of colors such as C (cyan), M (magenta), Y (yellow), and K (black), respectively.
In the image formed by such a multicolor printing, there is a boundary portion at which two different colors are adjacent each other. For example, in such an image that an M color figure and a K color figure are allocated as shown in FIG. 10A, there are boundary portions 201 and 202 (which are indicated by the thick lines in FIG. 10A). In cases where when this image is transferred from press plates to a printing paper, a drift occurs at the position of the press plates corresponding to respective colors, or expansion and contraction occur in the printing paper. Hence there may arise disadvantages that a gap is left at these boundary portions and the paper color of the printing paper peeps out and the like.
In order to avoid such a gap, “trapping process” has heretofore been performed in a prepress step. Trapping process includes allocating, along a boundary portion at which two colors are adjacent each other on an image, a narrow figure (hereinafter referred to as a “trap figure”) composed of a color containing either or both color components of the two colors.
Recently, the editing process of images in the prepress step is often performed on a computer. A series of steps relating to trapping process are also implemented by executing a predetermined program on a computer.
In the editing process of images on a computer, in some cases, an image (page) printed by multicolor printing is handled as data that is configured by allocating a plurality of figures to predetermined positions. Between the plurality of figures, the order of relative upper and lower positions (the front and the rear on the page space) are designated. In cases where figures are allocated so as to overlap with each other, the figure to be displayed on the forefront is determined according to the order.
For example, the image as shown in FIG. 10A can be handled as data in which a K color annular shape 102 is allocated so as to overlap the upper side of an M color rectangle 101, as shown in perspective in FIG. 10B.
Trapping process of the image so configured is executed in units of figures constituting the image. A trap figure was allocated to all boundary portions existing between an objective figure of trapping process (hereinafter referred to as an “objective figure”) and lower figures.
For example, in the image as shown in FIG. 10A, when the annular shape 102 is an objective figure, a trap figure containing either or both color components of the M color and the K color is allocated to both of the boundary portions 201 and 202 existing between the objective figure and the lower rectangle 101.
However, such a trapping process might generate a trap figure containing any unwanted color component when a figure is allocated to a further upper side from the objective figure.
This problem occurs, for example, as shown in perspective in FIG. 11B, in cases where a K color annular shape 102 is overlapped on the upper side of an M color rectangle 101, and on a further upper side thereof, a C color rectangle 103 is overlapped so as to contact with the inside edge of the annular shape 102, thereby configuring an image 100 as shown in FIG. 11A. If in the data of this image, the above-mentioned trapping process is executed with the annular shape 102 as an objective figure, a trap figure containing either or both color components of the M color and the K color is allocated to both of boundary portions 201 and 202 existing between the objective figure and the lower rectangle 101, as described above. Unfortunately, in the after-configuration image shown in FIG. 11A, the boundary portion 202 is the portion that must in appearance become a boundary between the K color and the C color. Therefore, the allocation of a trap figure containing the M color component results in an unnatural color allocation.
Especially, in such an image 300 as shown in FIG. 12, expressing a gradation of color density in such a manner that a plurality of figures 302 to 304, which are of the same color but different only in degree of color density, are adjacent each other on the upper side of a figure 301 of a color different from their respective colors, the allocation of a trap figure containing the color component of the figure 301 to boundary portions between the figures 302 to 304 results in a fatal defect in appearance.